This invention relates to a method of apparatus for treatment of waste materials. It is particularly concerned with the treatment of "Sulphate Waste Material". By the term "Sulphate Waste Material" as used herein we mean waste material comprising sulphuric acid and/or a sulphate (typically an inorganic sulphate). Examples of such sulphate waste material include petroleum refinery sludge, spent pickling liquor and other wastes from chemical or metallurgical processes in which sulphuric acid is employed.
One particular example with which this invention is concerned is the sulphate waste material that is typically obtained in the manufacture of titanium dioxide pigments by the sulphate route. The sulphate waste material from this process particularly comprises relatively dilute sulphuric acid including dissolved iron sulphate. Typically, such sulphate waste includes from 14 to 20% by weight of sulphuric acid and up to 20% by weight of iron sulphate may be included, and typically traces of other inorganic sulphates, the balance being water.
It has been proposed to treat this waste material by subjecting it to spray drying whereby to remove the water without liberating a substantial amount of sulphur dioxide. The resultant anhydrous sulphate salts are then removed from the acid and the acid is recondensed at a higher strength. One disadvantage of this method is that it requires relatively complex spray drying equipment and the spray drying stage is difficult to optimise. A second and more fundamental disadvantage is that in the event that the waste material includes metal ions that may have a deleterious effect on the titanium dioxide pigment, such metal ions may be carried with the acid vapour with the result that the resulting concentrated sulphuric acid may not be of sufficient purity for reuse in the manufacture of titanium dioxide.
In our UK patent application 2181120A we disclose a process for the treatment of sulphate waste material that includes a significant proportion of inorganic sulphate in which the sulphate waste material may be cracked in a rotary or other kiln or in a fluidised bed. In the fluidised bed, an oxygen-rich gas such as pure oxygen or oxygen-enriched air is preferably employed to fluidise the bed, and a solid fuel such as coke may be burned to raise the temperature necessary to crack the sulphate waste material fed into the bed. With this method the oxygen-rich gas typically fulfils two functions. The first function is that of providing the necessary oxygen for burning the fuel and thereby generating a temperature sufficiently high to effect the thermal cracking of the sulphate. The second function is to fluidise the bed. Although in some examples, it is perfectly possible for the oxygen-rich gas to fulfil both these functions, it is to be appreciated that if undesirably high levels of residual oxygen are to be avoided in the gas leaving the fluidised bed, in other examples it will be necessary for the oxygen-rich gas to contain a relatively high proportion of nitrogen so as to obtain an adequate fluidising velocity and a stage may be reached where the proportion of nitrogen becomes so high that it is no longer possible to obtain the optimum temperature for thermal cracking or the overall requirements for thermal energy become undesirably high.
The invention relates to an alternative method and apparatus that overcomes or reduces these problems.